The present invention relates to the art of nuclear medical imaging. It finds particular application in conjunction with rotating one-dimensional (1D) slat-collimated gamma cameras and single photon emission computed tomography (SPECT), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications and other diagnostic imaging modes such as, e.g., positron emission tomography (PET).
In diagnostic nuclear imaging, one or more radiation detectors are mounted on a movable gantry to view an examination region which receives a subject therein. Typically, one or more radiopharmaceuticals or radioisotopes such as 99mTc or 18F-Fluorodeoxyglucose (FDG) capable of generating emission radiation are introduced into the subject. The radioisotope preferably travels to an organ of interest whose image is to be produced. The detectors scan the subject along a selected path or scanning trajectory and radiation events are detected on each detector.
In a traditional scintillation detector, the detector includes a scintillation crystal that is viewed by an array of photomultiplier tubes. A collimator which includes a grid- or honeycomb-like array of radiation absorbent material is located between the scintillation crystal and limits the angle of acceptance of radiation which will be received by the scintillation crystal. The relative outputs of the photomultiplier tubes are processed and corrected to generate an output signal indicative of the position and energy of the detected radiation. The radiation data is then reconstructed into an image representation of a region of interest.
A so-called rotating laminar emission camera (xe2x80x9cROLECxe2x80x9d), also know as the rotating laminar radionuclide camera, has been disclosed in the literature and prior art. Devices utilizing a cadmium telluride (CdTe) detector arrangement have been disclosed in Mauderli, et al., A Computerized Rotating Laminar Radionuclide Camera, J. Nucl. Med 20: 341-344 (1979) and Entine, et al., Cadmium Telluride Gamma Camera, IEEE Transactions on Nuclear Science, Vol. NS-26, No. 1: 552-558 (1979). According to one version, the device included a linear array of CdTe detectors separated by tungsten plates. The plates were arranged perpendicular to the detector faces and confined the field of view of each detector to one dimension. The device had a square (approximately 4 cmxc3x974 cm) active area, although a circular lead mask reduced the active area to 13.2 cm2. The detectors, which had platinum-film electrodes, were attached to copper strips on a printed circuit board that also served as the base of the collimator and as a support for amplifier-discriminator circuits.
A ROLEC having a 250 mmxc3x97250 mm active area was disclosed in U.S. Pat. No. 4,090,080 to Tosswill, issued May 16, 1978, incorporated by reference herein, in its entirety. The device included scintillating plastic sheets disposed between parallel collimator plates supported by a steel frame in a perpendicular orientation with respect to the radiation receiving face of the detector. Fiber optics epoxied to the rear surface of each scintillating sheet transferred light generated in the each of the detectors to a corresponding photomultiplier. According to Tosswill, the ROLEC may be operated moving its axis along another curved or other configuration or without rotation, with symmetry preferred but not essential.
Devices using a segmented germanium crystal have been described by Urie, et al., Rotating Laminar Emission Camera with GE-detector, Med. Phys. 8(6): 865-870 (1981); 20 Mauderli, et al., Rotating Laminar Emission Camera with GE-Detector: An Analysis, Med. Phys. 8(6): 871-876 (1981); Malm, et al., A Germanium Laminar Emission Camera, IEEE Transactions on Nuclear Science, Vol. NS-29, No. 1: 465-468 (1982); and Mauderli, et al., Rotating Laminar Emission Camera with GE-detector: Further Developments, Med. Phys. 14(6): 1027-1031 (1987).
In a first version, a 11.5 mm thick, 45 mmxc3x9745 mm segmented germanium detector was placed behind parallel tungsten plates oriented perpendicular to the face of the detector. The crystal was segmented to form a plurality of channels, with the plates aligned with the segmentations. A 4.5 cm diameter viewing aperture was located between the detector and the activity source. Projection data acquired at multiple angular orientations as the detector-collimator assembly was rotated about its center was mathematically reconstructed to form a two-dimensional (2D) image of the activity distribution.
A second version simulated a 195 mmxc3x97195 mm detection area using five germanium blocks having a total length of 250 mm segmented into distinct electrical channels. The detector was translated linearly in a direction perpendicular to the plane of the plates to simulate a full-size detector.
While maintaining certain advantages, such as a better sensitivity-resolution compromise, over, e.g., traditional Anger cameras, the previously developed ROLECs are burdened by some other undesirable limitations. For example, the type of one dimensional collimation or slat geometry used by ROLECs presents issues with the image reconstruction. In particular, the ROLEC geometry results in a plane integral reconstruction problem as opposed to the line integral reconstruction problem that is generally encountered in traditional Anger camera applications. Moreover, the geometry produces a plane integral only in a first approximation.
In actuality, the plane integral should have a weighting factor introduced thereto to account for the fact that a detector""s sensitivity has a 1/r dependence to an object being imaged, where r represents the distance of a radiation event under consideration to the detector. That is to say, the detector is generally more sensitive to relatively close objects and less sensitive to far away objects. Previously developed ROLECs merely disregard or ignore the 1/r weighting factor in solving the reconstruction problem. In previously developed ROLECs, the first approximation is merely accepted, i.e., it is accepted that the geometry produces plane integrals without 1/r weighting. Ultimately, failure to model this 1/r weighting factor or dependence, or improperly modeling the same, reduces the quality of images produced.
Additionally, while ROLECs have the advantage of relatively higher efficiency and spatial resolution, they have been expensive to produce inasmuch as significant quantities of relatively expensive detector material have been required. Although detector material cost can be reduced by using a number of relatively smaller detector segments, such an approach complicates the manufacturing process and requires that variations in the response of the individual segments be considered. Still another drawback is that the collimator slat length has been equal to the detector field of view. This undesirably results in: additional detector, collimator, and structural materials being used; introduction of spurious counts which do not contribute to useful image information; and, additional mass and bulk being incorporated into a rotating structure.
The issues raised in the foregoing paragraph have been addressed, at least in part, by developing a ROLEC which utilizes a detector area which is small compared to the length of the collimator slats. See, e.g., commonly owned U.S. patent application Ser. No. 09/206,508 of Gagnon, et al., filed Dec. 7, 1998, incorporated by reference herein, in its entirety. However, when the detector area is small compared to the length of the collimator slats, the 1/r weighting issue is exacerbated.
The present invention contemplates a new and improved ROLEC and reconstruction technique therefor which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a rotating laminar emission camera includes a detector having a side which detects radiation facing an object being studied. The detector rotates about a central axis perpendicular to the side of the detector facing the object. A collimator constructed of a radiation attenuative material is arranged on the side of the detector facing the object. The collimator including a plurality of spaced apart slats which are tilted at an angle greater than zero with respect to the axis.
In accordance with a more limited aspect of the present invention, the tilt angle of the slats is inversely proportional to the camera""s resolution.
In accordance with a more limited aspect of the present invention, the tilt angle of the slats is greater than zero degrees and less than or equal to 5 degrees.
In accordance with a more limited aspect of the present invention, the tilt angle of the slats is approximately 0.25 degrees.
In accordance with a more limited aspect of the present invention, the slats are thin rectangular members with two rounded corners at opposing longitudinal ends of each slat. The two rounded corner are opposite an edge of the slats proximal to the side of the detector facing the object.
In accordance with a more limited aspect of the present invention, the two rounded corners have a radius of curvature approximately equal to a height of the slats, where the height of the slats is defined by a distance that the slats extend outwardly from the side of the detector facing the object.
In accordance with a more limited aspect of the present invention, the slats extend longitudinally beyond an outer dimension of the side of the detector facing the object.
In accordance with a more limited aspect of the present invention, the detector includes a plurality of detector segments which are arranged to be visible through the slats of the collimator such that each detector segment detects radiation received through a respective gap formed by adjacent slats.
In accordance with a more limited aspect of the present invention, the slats extend longitudinally beyond an outer dimension of the detector segments.
In accordance with a more limited aspect of the present invention, the slats are parallel to one another.
In accordance with a more limited aspect of the present invention, the tilt angle of the slats progressively changes for each slat.
In accordance with another aspect of the present invention, a nuclear medical imaging apparatus includes a receiving region wherein an object being imaged is received, and a radiation detector having a side which faces the receiving region. The detector includes a collimator fabricated from radiation attenuative material arranged on the front thereof between the detector and the receiving region. The collimator is made up of a plurality of spaced apart slats which are tilted at an angle greater than zero with respect to a central axis perpendicular to the side of the detector which faces the receiving region. A drive imparts about the central axis relative rotation between the detector and the object being imaged.
In accordance with a more limited aspect of the present invention, the nuclear medical imaging apparatus further includes image reconstruction means employing a Radon transformation to reconstruct an image representation of the object from data corresponding to radiation detected by the detector. The image reconstruction means approximates a first derivative of the Radon transformation by subtracting from data acquired at a first relative rotational orientation between the detector and the object, data acquired at a second relative rotational orientation between the detector and the object. The first and second relative rotational orientations are 180 degrees apart.
In accordance with a more limited aspect of the present invention, the slats are thin rectangular members with two rounded corners at opposing longitudinal ends of each slat. The two rounded corner are opposite an edge of the slats proximal to the side of the detector facing the receiving region.
In accordance with a more limited aspect of the present invention, the slats extend beyond an outer dimension of the side of the detector which faces the receiving region.
In accordance with a more limited aspect of the present invention, the radiation detector includes a plurality of detector segments which are arranged between the slats of the collimator such that each detector segment detects radiation received through a respective gap formed by adjacent slats.
In accordance with another aspect of the present invention, a method of nuclear medical imaging includes exposing a side of a radiation detector to an object being imaged such that a direction the detector is facing is defined by an axis perpendicular to the side of the detector. The method further includes varying about the axis a relative rotational orientation between the object and the detector. A first data set is obtained based on radiation detected by the detector at a first relative rotational orientation between the object and the detector, and likewise a second data set is obtained based on radiation detected by the detector at a second relative rotational orientation between the object and the detector. The second relative rotational orientation is displayed from the first relative rotational orientation. Finally, the second data set is subtracted from the first data set to determine a difference between the first and second data sets.
In accordance with a more limited aspect of the present invention, the first and second relative rotational orientations are displayed from one another by 180 degrees.
In accordance with a more limited aspect of the present invention, the difference obtained is used as an approximation of a first derivative of a Radon transformation.
In accordance with a more limited aspect of the present invention, the method of nuclear medical imaging further includes taking a derivative of the difference obtained, and backprojecting the derivative of the difference.
In accordance with a more limited aspect of the present invention, the first and second data sets represent 1/r weighted planar integrals, where r represents a distance from a detected radiation event to the detector.
In accordance with a more limited aspect of the present invention, the first and second data sets are obtained in such a manner that the 1/r weighting factor becomes negligible when the difference is obtained.
In accordance with a more limited aspect of the present invention, the method of nuclear medical imaging further includes collimating radiation received by the detector such that radiation received by the detector is substantially limited along one direction to a reception trajectory having an angle greater than zero with respect to the axis.
One advantage of the present invention is high image quality resulting from proper modeling of the measured radiation data.
Another advantage of the present invention is an improved resolution-sensitivity compromise compared to traditional Anger cameras.
Yet another advantage of the present invention is accurate reconstruction of a local region within the detector""s field of view without radioactivities in outside regions affecting the image values in the particular local region.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.